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Transcript
Can environmental factors acting on an organism cause inherited changes in
phenotype in its descendants?
It is now common knowledge that DNA is the principle hereditary material that
determines an organisms phenotype and is what gets passed down to its descendants. This
means that all the hereditary information is coded for by the DNA and that any thing the
organism may do in their lifetime or any environmental change they may undergo will
not be passed down to its offsprings. However, as molecular biology is advancing, some
researchers have proved that factors other than changes in the genes can cause inherited
changes in phenotype in an organisms descendants. Such an area of biology have been
designated as Epigenetics.
Epigenetics as understood in modern biology is “the study of heritable changes in gene
function that occur without a change in the sequence of nuclear DNA”.1 this refers to any
modification in the genes other than the change in the DNA sequences itself. It includes
how environmental factors acting on an organism can change the way genes are
expressed in the offspring. And how processes such as gene regulation are involved
during the development of an organism.
Gene regulation is when the level of gene expression can vary under different conditions.
Each of our cells carry the genes which code for everything needed to sustain the body,
but only some of them are active. This is because the epigenetic modifications act like
switches, turning on the gene that is most needed in a particular cell at a particular time,
so helping to control gene activity. In doing so, there is a memory of gene activity that
can be passed on each time a cell divides.
One of the well known epigenetic signal is DNA methylation which tags cytosine with a
methyl group. It is generally associated with silencing gene expression because active
genes are usually unmethylated. Another important epigenetic mechanism is chromatin
remodelling. Chromatins are densely packed structure of coiled up DNA wrapped around
proteins called histones inside the nucleus. These histones can be chemically modified by
adding acetyle, methyl or phosphate groups thereby altering chromatin structure. This can
then influence the activity of nearby genes, most probably switching them on. Together,
these mechanisms regulate gene expression throughout the genome and may help to
explain unusual occurrences such as parental ‘imprinting’.
Parental or Genomic imprinting is an example of epigenetic inheritance. The word
imprinting implies a type of marking process that has a memory. Similarly, genomic
imprinting is when a segment of DNA is marked and that mark is retained and recognised
throughout the life of the organism inheriting the marked DNA. The marking process
causes the offspring to distinguish between maternally and paternally inherited alleles.
This can sometimes lead to the offspring expressing one of the two alleles but not both,
this process is known as monoallelic expression.
At the molecular level, imprinting has been known to involve DNA known as
Differentially Methylated Regions (DMRs) that are found near the imprinted genes.
Depending on the type of genes, these regions are methylated in the oocyte or the sperm
but not both. These DMRs contain binding sites for one or more proteins that regulate the
transcription of nearby genes. Most probably, methylation at a DMR will result in an
inhibition of gene expression. Due to this, imprinting is usually described as a marking
process that silences gene expression by preventing transcription.
Imprinting plays a key role in the inheritance of certain human diseases such as PraderWilli syndrome (PWS) and Angelman syndrome (AS). Patients suffering from PraderWilli syndrome have reduced motor function, obesity and mental deficiencies. Whereas
Angelman syndrome patients are hyperactive, have unusual seizures and repetitive
symmetrical muscle movements and show mental deficiencies. Both syndromes involve a
small deletion in human chromosome 15 (locus 3), if this deletion is inherited from the
maternal parent, it leads to Angelman syndrome, paternal inheritance leads to PraderWilli syndrome. Research has shown that this region contains closely linked but distinct
genes that are maternally or paternally imprinted.
Consequently, research in genomic imprinting as well as other epigenetic inheritance
mechanisms not only show that genes have switches that can regulate gene expression
but that these ‘switches’ can also be passed down to succeeding generations. This means
that a 'memory' of an event could be inherited since a simple environmental effect could
switch genes on or off .
A recent finding has revealed that environmental factors affecting a particular generation
can have consequences that can be passed down to a later generation. The research has
shown that a famine that has occurred in the lives of the grandparents can cut short the
life expectancy of their grandchildren. One such research2 was conducted in a German –
imposed food embargo in western Holland during a famine in winter. They have linked
prenatal exposure to famine to a range of developmental and adult disorders such as
diabetes, obesity, coronary heart disease, cancer and birth of smaller-than-normal grand
children.
Such findings have led some people to claim “you are what your grandmother ate”3. such
bold statements may come as a shock to the scientific community in the 21st century
because it remind us of Lamarckian inheritance of acquired characteristics. The long
refuted Lamarckian theory stated that when an organism acquired a certain characteristic
in their lifetime, and he used the example of giraffes stretching their necks to reach
treetops, they would give birth to progeny with similarly stretched necks. Although
epigenetic inheritance isn’t necessarily bringing this theory back, but it is giving reason to
reconsider how environmental cues could cause phenotypic changes transmittable to
offspring."He had a basically good idea but a bad example," (Rohl Oflsson, Uppsala
University, Sweden)
Some scientists on the other hand, had alternative ideas to explain epigenesis. The
psychologist Erik Erikson4 developed an epigenetic theory to explain human
development on the basis of psycho-social crisis. He believed that every individual in
their life goes through several developmental stages which are marked by a crisis.
According to this theory, although these stages are largely predetermined by genetics, the
way in which the crisis is overcome are not. In otherwords, the environmental factors are
largely responsible for causing the changes in an organisms phenotype.
Baldwinian evolution is another theory developed to explain how organisms can pass
learned abilities to their offspring. It is said to be an intermediate between natural
selection and social adaptation. It explains that as well as inheriting the genes encoded to
by the DNA, through non specific selection the selected offspring would have an
increased capacity for learning new skills as well as skills learned by the their ancestors.
It may be a crucial factor for explaining the rapid evolution of the human mind which as
many believe cannot simply have been the product of mere natural selection.
Therefore, when it comes to unusual phenomena, such as when identical twins who share
the same DNA and often the same environment can sometimes be so different ( for
instance if one is diagnosed with schizophrenia while the other is fine), simple genetic
can fail to give an answer. A better explanation can indeed be offered by epigeneticists
who say that the differences between identical twins are due to changes in the genome
that don’t effect the DNA sequences.
Perhaps epigenetics is an adaptive mechanism through which humans and many other
organisms may have adapted to change in the environment. It seems to provide a means
by which organism can react to environmental ques. This can even be advantageous to
the offspring since it enables it to adapt better to the environment.
The exact extent to which epigenetics affects the phenotype of an organism may not still
be known but what is for sure is that epigenetics is here to stay as more and more
evidences accumulate to suggest that a simple DNA to RNA to PROTIENE association is
not enough to explain heredity. Such evidence seem to prove that environmental factors
acting on an organism can cause inherited changes in phenotype in its descendants.
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